From the employer's perspective, the exit interview is primarily about spelling out specific ongoing obligations. For instance, this is when you'll hear fairly obvious things like "If you do need to come to the office for any reason, you need to be signed in", or polite reminders like "You are not allowed to divulge any trade secrets, even to your new employer". Normally you'll be asked to sign something specific HR needs. You may have agreed to things, such as supporting the company even after you leave in litigation, and you'll be asked to sign a reminder.
Some facts may be established, such as you leaving on your own accord, which affects unemployment benefits you may try to claim later.
Sometimes HR wants to cull information about your experiences for their benefit, but it is not in your best interest to tell the truth. I suspect HR rarely gets useful information from these in that respect.
If there is a dispute later, you're likely to be portrayed as leaving without fulfilling your obligations, which can undermine claims for compensation, etc.
You should be paid for the day, or at least the time.